Add a setting allowing to append SHA-1 to the result artifact name

Following change adds a new SettingKey: `appendContentHash`. This setting can be set to true for assembly, assembly-package-dependency and assembly-scala tasks. When it's enabled, the result assembly .jar file has additional **SHA-1** fingerprint appended. This fingerprint is calculated basing on the content of all files included in the assembly.
This is extremely useful when you deploy your dependencies separately. You can now verify if anything has changed and ignore uploading of big 'fat jar' if SHA-1 is the same.